So you would suggest I train Luna (3 year old) and Mia (puppy) because they are the leaders? Yes, I would, definitely!
Are they the instigators? I would work on them first.
How do I get them to quiet down? Pennies in a can has always worked for me. I learned about it on this site. Get a soda can or something of the like and whenever they bark, shake it loudly and say "NO" or "quiet!" They do not like the sound. Eventually, you should just be able to say "quiet!" without the can. Spray bottles can work well too, just a little squirt in the face. They usually don't like that.
How do I introduce them to people and not have them bark or nip? Have guests/friends/etc give them a treat. Let your dogs know these people mean no harm. They are probably barking and nipping because they think they are protecting you. YOU need to protect them.
Does walking more help? YES! Dogs thrive on a good walk. Gives them new environment, new smells and sights.. gets them some interaction. And it majorly tires them out equaling less barking.
And when they walk and bark, what should I do? Either shake the can, Like I said. Or when you're out on a walk, if you don't wanna look like a fool shaking a can, LOL.. just turn the other direction. Let them know you are NOT going to allow them to move forward until they are quiet. Out of sight, out of mind type of thing. The minute they bark, a quick tug on the leash and turn in the other direction until they stop barking.
It will take ALOT of time, consistancy and patience with six of the little rascals! I think owning a lot of dogs is great. But you've really got to consider the responsibilties that come along with owning six dogs. If you can't devote a walk to all of them every day... I would say re-homing one or two should be considered a possibilty. It's not fair to the dogs. From the way you describe it, they are NOT happy. That constant barking, nipping, etc means they are very stressed out dogs and very concerned with trying to be the leader and protector. Take that weight off of their shoulders and they will be much happier!